If sasuke gets to have Izanami, my only concern for the technique's use will be that while it is able to turn illusions into reality it should be limited to using it on the enemy and no one's self.
If Izanagi makes whatever is supposed to happen to the wielder in reality an illusion, then Izanami should be able to change that Illusion back to reality making it a perfect counter. This two Jutsu's can nullify or cancel each other out and allow for both parties to fight a "real battle".
Although I'm compelled to add that like sasuke mentioned the shinobi wielding the weapon determines the extent to which it can be useful thus the wielders of both Izanagi and Izanami can outdo each other depending on skill, stamina and chakra levels.
If sasuke awakens this power as the 4th MS power due to the acquisition of his EMS, it will be the power that helps both Sasuke and Naruto to win against Madara. thus If Izanami should work like I mentioned.